1. Field of the Invention
The present invention relates to an XY stage which is suitable for a wire bonder, and more particularly to an XY stage which prevents a vibration caused by a high-speed movement and improves the controllability.
2. Description of the Related Art
When a large-scale integrated circuit (LSI) or a semiconductor device is connected to a lead frame using gold wires or the like, a wire bonder having an XY stage has been used. Such an XY stage is required to realize a high-speed and highly accurate positioning and to have a low level of residual vibration after driving/stopping.
FIG. 1 is a schematic diagram illustrating a structure of a conventional wire bonder stage. FIG. 2 is a schematic diagram showing that a bonding head is attached onto the conventional wire bonder stage shown in FIG. 1.
In the conventional wire bonder stage, two X-axis guides 102 are provided on a square-plate-shaped base 101. The two X-axis guides 102 extend in a direction parallel to each other. This direction is referred to as an X-axis direction. An X-axis table 103 is provided on the X-axis guides 102. X-axis follower sections (not shown) for following the X-axis guides 102 are provided on the lower surface of the X-axis table 103. The X-axis table 103 can be moved in the X-axis direction. Two Y-axis guides 104 extending in a Y-axis direction which is perpendicular to the X-axis direction are provided on the upper surface of the X-axis table 103. A movable table 105 is provided on the Y-axis guides 104. Y-axis follower sections (not shown) for following the Y-axis guides 104 are provided on the lower surface of the movable table 105. The movable table 105 can be moved in the Y-axis direction and also in the X-axis direction because of the X-axis table 103 and the X-axis guides 102. The upper surface of the movable table 105 is a plane surface, and a bonding head 110 is fixed thereto.
An X-axis voice coil motor 106 (hereinafter, a voice coil motor is referred to as a VCM) is disposed so as to be adjacent to the movable table 105 in the X-axis direction. A Y-axis VCM 107 is disposed so as to be adjacent to the movable table 105 in the Y-axis direction.
The X-axis VCM 106 includes a yoke section 106c with a shape of a rectangular tube. The yoke section 106c is fixed to a support (not shown), and has an opening passing therethrough in a lateral direction. An iron core 106d is provided so as to divide the opening into two sections at an intermediate height of the yoke section 106c. The X-axis VCM 106 also includes an X-axis movable element 106a which is connected to the movable table 105, and a coil 106b which is wound around the X-axis movable element 106a. The coil 106b is wound so as to surround the iron core 106d. The lateral length of the coil 106b is about equal to or greater than a movable range of the movable table 105. The X-axis movable element 106a and the coil 106b can be moved without a constraint in the Y-axis direction. Magnets (not shown) are attached to the inner upper surface and inner bottom surface of the yoke section 106c so as to make a magnetic circuit which generates a magnetic field inside and around the coil 106b. 
Likewise, The Y-axis VCM 107 includes a yoke section 107c having the shape of a rectangular tube. The yoke section 107c is fixed to the support, and has an opening passing therethrough in a lateral direction. An iron core 107d is provided so as to divide the opening into two sections at an intermediate height of the yoke section 107c. The Y-axis VCM 107 also includes a Y-axis movable element 107a which is connected to the movable table 105, and a coil 107b which is wound around the Y-axis movable element 107a. The coil 107b is wound so as to surround the iron core 107d. The lateral length of the coil 107b is about equal to or greater than the movable range of the movable table 105. The Y-axis movable element 107a and the coil 107b can be moved without a constraint in the X-axis direction. Magnets (not shown) are attached to an inner upper surface and an inner bottom surface of the yoke section 107c so as to make a magnetic circuit which generates a magnetic field inside and around the coil 107b. 
The movable table 105 and the Y-axis movable element 107a together constitute an upper stage section 108, and the X-axis table 103 and the X-axis movable element 106a together constitute a middle stage section 109.
The mass of the bonding head 110 is greater than that of each of the upper stage section 108 and the middle stage section 109. As a result, when considering the bonding head 110 and the upper stage section 108 which are integrated with each other as a single member, the overall center of gravity of the bonding head 110, the upper stage section 108, and the middle stage section 109, which are members capable of moving in the X-axis and Y-axis directions, coincides approximately with the center of gravity of such an integrated member.
In the thus-structured conventional wire bonder stage, the middle stage section 109 follows the X-axis guides 102, thereby being guided in the X-axis direction with respect to the base 101. The upper stage section 108 follows the Y-axis guides 104, thereby being guided in the Y-axis direction with respect to the X-axis table 103, and is movable also in the X-axis direction together with the middle stage section 109.
Such an XY stage which two-dimensionally drives the movable section externally and indirectly via guides has a low level of positioning accuracy. In order to solve such a problem, there has been proposed an XY stage which directly positions a movable table (Japanese Patent Laid-Open Publication No. Hei 1-291194).
According to the XY stage described in this publication, two linear motors are provided on a base, and a coil of each of the linear motors is directly connected to the lower surface of a movable table. An L-shaped X-axis stage, which is guided in the X-axis direction by an X-axis guide rail and an X-axis linear guide, and the movable table are provided on the same plane. A straight line section of the X-axis stage functions as a Y-axis guide rail, and the movable table is guided in the Y-axis direction by the Y-axis guide rail and a Y-axis linear guide. A plane bearing is provided between the X-axis stage and the movable table, and the base. In this manner, the movable table is driven directly by the linear motors rather than via guides.
An XY stage including a link mechanism for holding a movable table in parallel to a base surface to realize a high-speed movement has been proposed (Japanese Patent Laid-Open Publication No. Hei 11-148984).
In the XY-stage described in this publication, two VCMs whose coils are fixed to the movable table are provided. A plurality of links are connected to the movable table, and the surface of the movable table is held in parallel to the surface of a base while the movable table is lifted up from the base. With such a structure, a single table is sufficient, thereby making it possible to downsize the XY stage itself.
According to the conventional wire bonder stage as shown in FIG. 1 and FIG. 2, the mass of the bonding head 110 is large as described above. Therefore, a moment is likely to occur during the operation, and thus the guides need to have a rigidity strong enough to withstand the moment. FIG. 3A and FIG. 3B show a change in the position of the center of gravity g of the movable section when the bonding head 110 is moved in the Y-axis direction. FIG. 3A is a schematic diagram showing the position of the center of gravity g before the movement, and FIG. 3B is a schematic diagram showing the position of the center of gravity g after the movement.
The bonding head 110 is generally heavier than each of the upper stage section 108 and the middle stage section 109, and total mass of the member composed of the bonding head 110 and the upper stage section 108 which are integrated with each other is greater than that of the middle stage section 109. As a result, when the upper stage section 108 and the bonding head 110 move in the Y-axis direction, the position of the center of gravity g of the movable section changes to a large degree as shown in FIG. 3A and FIG. 3B. Therefore, if the X-axis VCM 106 is driven under such a state, a driving force acts on a position significantly shifted from the center of gravity g of the movable section as viewed in a plan view. When the movable table 105 is moved in the X-axis direction, the position of the center of gravity g of the movable section changes in association with the position of the movable table 105 in the Y-axis direction. Therefore, a moment force is generated, causing a problematic vibration in a yawing direction. In the wire bonder, although a bonding step is performed immediately after the XY stage is stopped, the vibration as described above leads to a reduced positioning accuracy or the like, thereby significantly degrading the bondability. Thus, as described above, each of the X-axis guides 102 and the Y-axis guides 104 needs to have a rigidity strong enough to be able to hold a moment load due to the movement of the movable table 105 and the bonding head 110. Accordingly, the downsizing of the XY stage itself is difficult to achieve.
According to the conventional XY stage described in Japanese Patent Laid-Open Publication No. Hei 1-291194, since the height at which the VCM coil is positioned is different from the height at which the movable table is positioned, a driving force from the VCM does not act on the center of gravity of the movable section. Thus, it is difficult to sufficiently suppress the vibration caused by the high-speed movement of the movable stage.
Moreover, since a position detector is provided at a position above the movable table, the accuracy of the position detection is insufficient. Thus, position control may be difficult to perform.
Furthermore, according to the conventional XY stage described in Japanese Patent Laid-Open Publication No. Hei 11-148984, although the intended objective can be achieved, it is difficult to suppress the vibration in the yawing direction caused by the high-speed movement of the movable stage.
According to these conventional XY stages, since the vibration in the yawing direction occurs, a feedback control needs to be performed in order to reduce an influence due to the vibration. In addition, these conventional XY stages have the problem of a narrow control band.
Since the X-axis table 103, the Y-axis guides 104, the movable table 105, the Y-axis movable element 107a, and the coil 107b of the Y-axis VCM 107 are included among members on which a thrust from the X-axis VCM 106 acts, a very large VCM needs to be employed as the X-axis VCM 106 if a high thrust is required. Therefore, the force of inertia of the movable section as a whole becomes large, and the vibration in the yawing direction or the like which resonates at a low frequency becomes a problem.